This is a study of the pathophysiology of experimental immunologic glomerulonephritis in rats of the Munich-Wistar strain with surface glomeruli accessible to micropuncture techniques. Findings include: 1. Despite marked glomerular injury and reduction of the ultrafiltration coefficient to one-third normal, there was an adaptive increase in ultrafiltration forces and SNGFR and whole kidney GFR remained normal. 2. In the above model, filtration fraction, thus peritubular oncotic pressure falls. This was shown to be associated with a fall in absolute and fractional proximal reabsorption. 3. It was shown that permeability to neutral macromolecules (dextran) falls in this disease in rats, whereas permeability to charged macromolecules (dextran sulfate) is increased, explaining the observed proteinuria.